Electrochemical cells are presently the preferred method of providing cost effective portable power for a wide variety of consumer devices. The consumer device market dictates that only a handful of standardized cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) be provided. Moreover, more and more consumer electronic devices, such as digital still cameras, are being designed with relatively high power operating requirements. As has been the practice within the market, consumers often prefer and opt to use primary batteries for their convenience, reliability and sustained shelf life as compared to comparable, currently available rechargeable (i.e., secondary) batteries.
Within this context, it is readily apparent that design choices for primary (i.e., non-rechargeable) battery manufacturers are extremely limited. For example, the necessity of using specified nominal voltages significantly limits the selection of potential electrochemical materials, and the use of standardized cell sizes restricts the overall available internal volume available for active materials, safety devices and other elements typically expected in such consumer products. What's more, the variety of consumer devices and the range of operating voltages for those devices make smaller nominal voltage cells (which can be provided separately or in series, thereby giving device makers more design options) advantageous to higher voltage electrochemical pairings. Thus, 1.5 V systems, such as alkaline or lithium-iron disulfide systems, are far more prominent than others, such as 3.0 V and higher lithium-manganese dioxide. Correspondingly, the design considerations for each electrochemical systems (e.g., alkaline v. lithium-iron disulfide, etc.) are all significantly different.
Within the realm of 1.5 V systems, lithium-iron disulfide batteries (also referred to as LiFeS2, lithium pyrite or lithium iron pyrite) offer higher energy density, especially at high drain rates, as compared to alkaline, carbon zinc or other systems. However, current regulatory limitations on the amount of lithium in primary batteries make the FR03 (AAA LiFeS2 cells) and FR6 (AA LiFeS2 cells) sizes especially significant within the consumer market.
Other dissimilarities between lithium-iron disulfide and other chemical systems create other differences in the respective designs for such batteries. For example, alkaline and nickel oxy-hydroxide systems rely on an aqueous and highly caustic electrolyte that has a propensity for leakage, leading to very different approaches in terms of selection of internal materials and/or compatibility with containers and closures. In rechargeable 1.5 V systems (which do not include lithium-iron disulfide systems), various highly specialized electrochemical and/or electrolyte compositions may be used. Here, such high cost components are not a key design concern because secondary systems typically sell for a higher retail price than their primary battery equivalents. Moreover, the discharge mechanisms, cell designs and safety considerations are, by and large, inapplicable to primary systems.
But even with the inherent advantages of lithium-iron disulfide cells for high power devices (as compared to primary alkaline cells), LiFeS2 cell designs must still strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability, delivered capacity and intended use of the designed cell. Normally, low power designs emphasize the quantity of active materials, while high power designs focus more on configurations to enhance discharge efficiency. For example, a jellyroll design which maximizes surface area between the electrodes allows for greater discharge efficiencies but sacrifices capacity on low power and low rate discharges because such a design necessitates utilization of more inactive materials, such as separator and current collector(s) (both which occupy internal volume, thereby requiring removal of active materials from the cell design).
Notwithstanding the desire to improve discharge capacity, cell designers must also include and improve other battery characteristics, such as safety and reliability. Safety devices normally include venting mechanisms and thermally activated “shutdown” elements, such as positive thermal circuits (PTCs). Improvements to reliability primarily focus on preventing internal short circuits. In both instances, these characteristics ultimately require elements that occupy internal volume and/or design considerations that are usually counterproductive to cell internal resistance, efficiency and discharge capacity. Moreover, there are additional challenges because transportation regulations limit the percent amount of weight lithium batteries can lose during thermal cycling, meaning that cell designs for smaller container sizes like AA and AAA can only lose milligrams of total cell weight (usually by way of evaporation of the electrolyte). Plus, the reactive and volatile nature of the non-aqueous, organic electrolyte severely limits the universe of potential materials available (particularly with respect to interactions between the electrolyte and cell closure, separator and/or current collector(s) provided within the cell) as compared to other electrochemical systems.
Ultimately, maximizing the amounts of active materials in lithium-iron disulfide batteries may be the most difficult challenge. The basic, final electrochemical reaction for such cells is:4Li+FeS2→2Li2S+Fe
Because the reaction end products occupy more volume than the inputs, the electrode assembly swells as the battery discharges. In turn, swelling creates radial forces that can cause unwanted bulging of the cell container, as well as short circuits if the separator is compromised. Previous means of handling these problems include using strong (often thicker) materials for the cell housing and inactive components within the cell. However, thicker inactive materials limit the internal volume available and thicker, more rugged electrodes are not necessarily desirable in terms of performance because they allow for fewer winds possible in the jellyroll, resulting in less surface area between the electrodes and the potential for comparatively lower performance at higher drain rates.
A number of other approaches have been taken to strike an appropriate balance between optimal internal volume utilization and acceptable LiFeS2 cell capacity/performance. For example, a possible solution for problems created by swelling, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing one or more other active materials (such as CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2) with pyrite, although these additional materials can negatively affect the discharge characteristics of the cell, and the capacity and efficiency of the overall cell may also suffer.
Other means of improving discharge capacity in LiFeS2 cell contemplate the use of thinner separators and/or specific cathode coating mixes and coating techniques, as disclosed in U.S. Patent Publication Nos. 2005/0112462, filed on Nov. 21, 2003, and 2005/0233214, filed on Dec. 22, 2004, both conceived by the current inventor. Notably, as suggested by FIG. 2 of the '462 Publication, failure of the separator's physical integrity, which is dependent upon the tensile strength in both the web and cross web direction, and a resulting loss of a battery's capacity/utility occurs as the designed amount of electrode void volume decreases (expressed there as a function of jellyroll cross sectional void).
Also, U.S. Pat. Nos. 6,849,360 and 7,157,185 contemplate the use of a specific cathode coating formulation in combination with a set ratio of anode to cathode interfacial active materials (i.e., the theoretical interfacial input capacity ratio) in order to enhance cell performance.